Scanning acoustic microscopes are known in the art. After an initial effort disclosed by S. Sokolov in "An Ultrasonic Microscope," Dokl. Akad. Nauk., Vol. 64, pp. 333-335 (1949), the first demonstrated scanning acoustic microscope was described by R. A. Lemons and C. F. Quate in "Acoustic Microscopy by Mechanical Scanning," Appl. Phys. Letters, Vol. 24, pp. 163-164 (1974). A microscope using sound rather than light has two main advantages. First, the images are formed from information obtained by the interaction of sound waves with the specimen and, thus, contrast in acoustically generated images relates to the mechanical properties of the specimen. Actually, specimens which appear uniform or opaque under an optical microscope can produce a high contrast image when inspected with an acoustic microscope. Second, the spatial resolution of an imaging system depends on the wavelength of the illuminating radiation. In an optical microscope the greatest resolution is about 0.3 .mu.m in green light. Since the speed of sound wave propagation is very much less than the speed of light, only modest acoustic frequencies are necessary to obtain comparable acoustic wavelengths. Accordingly, an acoustic microscope has a potential for five to ten times better resolution than the optical limit.
The principles and the design of conventional acoustic microscopes have been extensively discussed in the literature, e.g. in V. Jipson and C. F. Quate, "Acoustic Microscopy at Optical Wavelengths," Appl. Phys. Letters, Vol. 32, pp. 789-791 (1978); J. E. Heiserman, "Cryogenic Acoustic Microscopy: The Search for Ultrahigh Resolution Using Cryogenic Liquids," Physica, Vols. 109 & 110B, pp. 1978-1989 (1982); and D. A. Sinclair, I. R. Smith and H. K. Wickramasinghe, "Recent Developments in Scanning Acoustic Microscopy," The Radio and Electronic Engineer, Vol. 52., No. 10, pp. 479-493 (1982).
Briefly, in a conventional scanning acoustic microscope, the specimen is translated point-by-point and line-by-line in a raster pattern past a focused diffraction-limited acoustic beam. The beam is generated by a piezoelectric transducer attached to the rear surface of a sapphire disk and centered on the axis of a single-surface spherical lens ground into the sapphire surface opposite the transducer. The specimen is translated in the focal plane of the lens, which is usually coated with an antireflection glass layer of one-quarter wavelength thickness. The lens and specimen are immersed in a body of liquid, such as water. The acoustic beam travelling down the sapphire is focused onto the specimen where it gets reflected and detected by the piezoelectric transducer. An electronic circulator serves to discriminate the reflected signal from the input signal. The output signal is used to modulate the brightness of a cathode ray tube display, whose x and y axes are synchronized with the scanning of the specimen. In order to produce acoustic wavelengths in the coupling liquid of less than a micrometer, the transducer is driven at frequencies in the gigahertz range. FIG. 1, published in several of the references cited above, shows the basic components of a prior art scanning acoustic microscope.
As already pointed out by P. Sulewski, D. J. Bishop and R. C. Dynes in "A Description of the Bell Laboratories Scanned Acoustic Microscope," The Bell Syst. Tech. Journ., Vol. 61, No. 9, p. 2174 et seq. (1982), the theoretical resolution of the acoustic microscope is proportional to the wavelength. Thus, an increase in the frequency should improve the resolution. However, for water and many other liquids, the acoustic attenuation is proportional to the second power of the frequency so that increasing the frequency only dramatically increases the power losses within the coupling liquid. In the interest of preserving the signal-to-noise ratio at an acceptable level, frequencies in the range of 3 GHz are presently considered optimal.
The resolving power of the conventional acoustic microscope is also limited by problems related to the acoustic beam diameter, in other words, to the diameter of the lens used. The smallest lens diameter found reported in the literature is 20 .mu.m (Sinclair et al., op. cit., p. 492). The resolution achievable with this lens was reported to be 220 nm at 1 GHz, with argon gas at 40 bar being used as the coupling medium.
A better resolution than is achievable with an optical microscope is not possible with a conventional acoustic microscope unless a monatomic gas at high pressure (such as argon) or a cryogenic liquid (such as liquid helium) is used as the coupling medium in place of water (Heiserman, op. cit.). While such a coupling medium will improving the resolution, it creates an ambient environment which restricts the use of the acoustic microscope to only the limited range of specimens suited for such an environment.